Coral limestone which is formed by accumulation of remains of coral, foraminifera, lime algae and mollusca occurs abundantly in the South Sea Islands, the Amami Islands, Tokunoshima Island, Okierabujima Island, Yoronto Island, and the Okinawa Islands. Although recently introduced into use for the foundation of road, it was unserviceable for any other use.
Based on the finding that coral fossil limestone has an excellent ability to adsorb heavy metals, the present inventor already accomplished the invention of U.S. patent application Ser. No. 367,357 now U.S. Pat. No. 3,898,224. Further researches have revealed that coral fossil limestone improves soils to give increased yields, evidencing its novel application as a soil improving agent or fertilizer.
Conventionally, finely divided limestone (CaCO.sub.3), quick lime (CaO), slaked lime (Ca(OH).sub.2) and magnesia-lime fertilizer are used in admixture with acid soils for soil improvement. However, these materials act merely to neutralize acidity and sharply elevate soil pH in proportion to the amount used, consequently involving an extremely limited tolerance in obtaining the desired neutral pH value. Furthermore they fail to act protractedly to a significant extent. In other words, they have a low ability to afford a sustained soil improving effect and are therefore difficult to use in adequate amounts. With excess amounts, they kill the plant. Quick lime irritates the skin and is hazardous to the respiratory organs, possibly producing adverse effects on man and animals exposed thereto.